This invention relates to automatic phase adjustment of a local oscillator signal in one channel of a QAM demodulator to assure a phase quadrature relationship between local oscillator signals in the two channels of the demodulator.
A QAM demodulator has in-phase and quadrature channels, each with its own phase detector receiving the QAM signal to be demodulated. The phase detectors also require local oscillator inputs having a phase quadrature relationship with respect to each other, that is separated in phase by ninety degrees. This quadrature relationship is very important. In a practical system it has been found necessary that the phase relationship of the local oscillator inputs to the detectors be within several hundredths of a degree of ninety degrees.
The basic quadrature phase shift required can be achieved by applying the output of the local oscillator through the phase port of a 0.degree./90.degree. hybrid to the phase detectors. However, several factors operate to cause deviation from the precise quadrature relationship required. For one thing, one of the alignment procedures for the demodulator acts directly to minimize intermodulation distortion, but affects the phasing of a local oscillator signal. In the long term, component aging and temperature impact the quadrature relationship of carrier and local oscillator signals in both the modulator and demodulator portions of a QAM system.